O-Hf isotope constraints on the origin of zircon in high-pressure melange blocks and associated matrix rocks from Tinos and Syros, Greece
O-Hf isotope constraints on the origin of zircon in high-pressure melange blocks and associated matrix rocks from Tinos and Syros, Greece
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DOI：
10.1127/0935-1221/2011/0023-2131
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发表时间：
2012-03
影响因子：
2.1
通讯作者：
B. Fu;Bence Paul;Bence Paul;J. Cliff;M. Bröcker;F. Bulle
中科院分区：
文献类型：
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作者：
B. Fu;Bence Paul;Bence Paul;J. Cliff;M. Bröcker;F. Bulle
大多数以前定年的锆石在高压混杂岩块和相关的基质岩石从蒂诺斯和锡罗斯（基克拉泽斯），希腊，产生离子微探针（SHRIMP II）U-Pb年龄约。80马。在许多情况下，尚不清楚锆石是火成岩还是变质/热液成因。氧和铪同位素比值的定年锆石已被确定，以进一步限制其形成模式。用离子探针（CAMECA IMS-1280）对变质辉长岩、榴辉岩、蓝闪岩、硬玉岩和绿片岩中的锆石进行了定点分析，得到了较大的δ 18 O值，其变化范围为2.0 ‰ ~ 7.6 ‰ VSMOW。而11个样品中绝大多数锆石的δ 18 O平均值在4.7 ‰ ~ 5.5 ‰之间，属于岩浆成因。这些值表明与现代洋壳的典型岩浆或与原始岩浆成分或地幔平衡的前体（5.3 ± 0.6 ‰，2 SD）的关系。δ 18 O（类地幔）和初始δ Hf值[δ Hf（t）= +10 ~+24]表明，80 Ma年龄的锆石为岩浆成因，结晶于亏损地幔的岩浆。扫描电子显微镜阴极发光成像显示，低δ 18 O锆石（颗粒或域;低至2.0 ‰），主要来自一个特殊的榴辉岩样品（平均4.3 ± 0.8 ‰，2 SD，n = 23），要么代表菜花状内部结构，要么代表外边缘的弱环带（或多孔）特征。以前的年龄测年锆石域与菜花状的内部结构表明，明显的年龄是相当年轻的（约。54、我和你57 Ma）比白垩纪的年龄确定的大多数锆石人口。总之，这些观察结果表明，低δ 18 O锆石（和三个高δ 18 O锆石，> 6.0 ‰）与一个有充分记录的始新世高压变质事件（约1000年）有成因联系。53-40 Ma），但不能完全排除岩浆后海底热液蚀变。
Most previously dated zircons in high-pressure melange blocks and associated matrix rocks from Tinos and Syros (Cyclades), Greece, yielded ion microprobe (SHRIMP II) U-Pb ages of ca. 80 Ma. In many cases it remains unclear whether the zircons are igneous or metamorphic/hydrothermal in origin. Oxygen and hafnium isotope ratios in the dated zircons have been determined to further constrain their mode of formation. Spot analysis of zircons from metagabbro, eclogite, glaucophanite, jadeitite and chlorite schists by ion microprobe (CAMECA IMS-1280) yields a large range in δ 18 O, varying from 2.0 ‰ to 7.6 ‰ VSMOW. The average δ 18 O values for most zircons in 11 samples however fall within a relatively small compositional range between 4.7 ‰ and 5.5 ‰, which is consistent with an igneous origin. These values suggest a relationship to magmas typical for modern oceanic crust or to precursors that had been in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6 ‰, 2 SD). The δ 18 O (mantle-like) and initial epsilon hafnium values [ɛ Hf ( t ) = +10 to +24] suggest that the 80 Ma old zircons are igneous in origin and crystallised from magmas that were derived from depleted mantle. Scanning Electron Microscope cathodoluminescence imaging indicates that lower-δ 18 O zircons (grains or domains; down to 2.0 ‰), mostly from one exceptional eclogite sample (average 4.3 ± 0.8 ‰, 2 SD, n = 23), either represent cauliflower-like internal structures or weakly zoned (or porous) features in outer rims. Previous age dating of zircon domains with cauliflower-like internal structures indicated apparent ages that are considerably younger ( ca. 54 Ma and ca. 57 Ma) than the Cretaceous age determined for the majority of the zircon population. Taken together, these observations suggest that the low-δ 18 O zircons (and three high-δ 18 O zircons, > 6.0 ‰) are genetically linked to a well-documented Eocene high-pressure metamorphic event ( ca . 53–40 Ma), but post-magmatic seafloor hydrothermal alteration cannot be completely ruled out.